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RPM Dependency Generator

One basic design goal of RPM is to be able to check if a package really is going to work on the system it is being installed. To achive this RPM does not (only) rely on requiring packages as stated by the packagers but auto detects executables and libraries that are used. To make this work RPM also examines the packages for libraries that could be used by other packages and creates "Provides:" for them. This works reasonable well for binary executables and shared object files (libraries). Where ever there is no automatic generation of Provides: and Requires: packagers can resort to add manual Requires: to the packages they need or even add Provides: to the packages by hand. Over the years this has led to huge areas that are not covered by RPM's auto generator. To claim back these areas a plug-in interface was created to allow packagers/distributions to auto create Provides: and Requires: for single domains without tampering with the rpm code.

File Attributes

To avoid passing every file through every dependency generator file attributes are used. All files in packages are classified based on the present file attribute rules, and files can have an arbitrary number of attributes. Each file attribute can have a Provides: and a Requires: generator. A file attribute is represented by a macro file in %{_fileattrsdir} (typically /usr/lib/rpm/fileattrs/), and must have .attr suffix to be processed. The following file attribute macros are recognized:

NAME needs to be replaced by the name choosen for the file attribute and needs to be the same as the file name of the macro file itself (without the .attr suffix). While technically all of them are optional, typically two or more of them are present to form a meaningul attribute. All the values are further macro-expanded on use, and additionally the path and magic related values are interpreted as extended regular expressions.

The path REs are matching the path of the file relative to the install root or in other word the path the files are going to end up on disk (e.g. /bin/bash). The magic REs are matched against the result of libmagic (see "man file" and "man magic"), and some of them are also stored in the FILECLASS tag (try {"{{rpm -q --fileclass PACKAGENAME}}}" for example). To get compatible results with rpm, use "file -z -e tokens <file>" when determining appropriate file attribute magic RE's. Matching is inclusive unless changed by flags: if path, magic or both match, the file is considered to have the attribute in question, unless there's a matching exclude pattern (also inclusive by default) or a flag which prevents the match.

Flags are a comma-separated lists, as of rpm 4.9.1 the supported flags are:

exeonly - require executable bit set

magic_and_path - require both magic and pattern to match

Generators

A generator is just an executable that reads file name(s) from stdin and writes out Provides: or Requires: on stdout. This way the generator can be implemented in whatever language is preferred and can use e.g. language specific libraries or tools. Generators get called once for each file with matching attributes. Generators can be declare in the file attributes file by defining the following macros:

%__NAME_provides
%__NAME_requires

The value is the command line of the generator script/executable and any arguments that should be passed to it. In addition to what's defined in the provides/requires macros, it's possible to pass additional arbitrary switches to generators by defining the following macros:

%__NAME_provides_opts
%__NAME_requires_opts

The _opts macros should not be used in file attribute definitions, they are intended for spec-specific tweaks only.
Note that any options are fully generator-specific, rpm only requires generators to support the stdin, stdout protocol.

Old Style Dependency Generators

Old style generators, also known as "the external dependency generator", differ from the "internal" one in several ways. One difference that generator developers need to be aware of is that the new generators get called once per each file of a type, but the old generator is passed the entire file list of a package all at once. For compatibility reasons all generators should accept arbitrary number of files on stdin. A more profound difference is the data generated: packages built with old-style generators contain less data about the files, such as "color" information which is vital for rpm's functionality on multiarch systems, file type information and per-file dependency tracking. The old-style generators are deprecated and should not be used for new packaging, this functionality is only kept for backwards compatibility and may get removed in a future release of rpm.

Writing Dependency Generators

Generally each type of Provides: and Requires should be handled in one place. The idea is not that every package ships it's own generators but one central package is taking care. So generators should go either directly into RPM (talk to us on the rpm-maint list) or shipped in one central package dealing with the domain. This would be something like the interpreter of the language in question or the package containing a central reqistry or even just the directory where other packages are supposed to put their files into.

TODO: Describe development model and decide upon a central repository for hosting and sharing generators that are not (yet) in RPM upstream.

Tweaking Dependency Generators

Technically all aspects of file attributes and the generator helpers they use can be overridden from spec by (re)defining the related macros, but packagers should generally avoid this as the attributes and their names are subject to change, depending on rpm version and which packages are present during build. Unwanted dependencies can be filtered with a separate set of macros which are intended primarily for use in spec-files:

The values of these are macro-expanded and the results interpreted as (extended) regular expressions. The exclude_from variants operate on paths, for example to prevent all provides generated from *.so files in %{_libdir}/mypkg/ from being added:

%define __provides_exclude_from ^%{_libdir}/mypkg/.*.so$

would prevent any provides from *.so files in %{_libdir}/mypkg/ from being added to packages. The exclude variants operate on generated dependency strings, for example the following would prevent all typical library requires from being added, regardless of which files they originate from:

%define __requires_exclude ^lib.*$

Note that within spec-files, any backslashes need to be double-escaped to prevent the spec parser from eating them.

Troubleshooting

rpmbuild and rpmdeps have a hidden --rpmfcdebug switch that enables additional output for the dependency generation stage. The output format is currently roughly as follows:

File attribute matches are the names of the fileattr rules which matched for the file, and that's where rule troubleshooting typically starts: a file with no attributes will not have any dependencies attached...

Open Questions

Passing the package version might be handy for interpreted languages that don't have support for versioned Requires: (like Python imports). This way the Provides: could get the version of the package and other packages could at least manually require a version (rage).